Cynthia Nixon was the clear winner of Wednesday’s Democratic gubernatorial debate, and not just because she had the easier job of offering lefty pie in the sky without the burden of ever having had to deliver on a campaign promise.

Gov. Andrew Cuomo did a remarkably poor job of defending himself against the attacks he had to know were coming, and even fumbled his own prepared slams.

She’s plainly well to his left, even though he’s been moving in that direction for years now. And in a Democratic primary, it doesn’t matter much that state-level single-payer health care and draconian rent control and letting public employees strike would all be disastrous in reality: Voters in the Sept. 13 primary will eat it up.

Yet he failed to even demand credit for his real progressive achievements — passing a $15 minimum wage, gay marriage and a family-leave law, for starters. Nor did he manage to (dare to?) explain that the Independent Democrats (mostly) didn’t give control of the state Senate to the Republicans, but simply traded some extra votes for GOP concessions on core Democratic issues.

And Cuomo fell utterly flat on corruption, basically moaning that he’d been betrayed by “smart people” who did “stupid things.” His only real defense on the subways, meanwhile, was to insist the city needs to pay its fair share — which doesn’t explain how the system fell into crisis on his watch.

Nixon’s attack lines stung: “He used the MTA like an ATM”; calling him a “corrupt corporate Democrat.” His attempted hits — mostly efforts to flag that fact she does business as a corporation, plus some reference to lobbying City Hall on behalf of Sarah Jessica Parker, and a very weird charge involving Michael Bloomberg — were too complicated, contrived and obscure.

If Cuomo can’t up his game, there won’t be any point in weaseling out of his promise not to join the 2020 presidential race.